The firm building Sydney's light rail is taking the NSW Government to court, claiming it is owed more than $1 billion extra, prompting Labor to call the project a "total embarrassment" for Premier Gladys Berejiklian.

Workers for subcontractor Acciona have been on a go-slow, with the project already running behind schedule by months.

The court action for $1.2 billion threatens to delay completion of Ms Berejiklian's pet project even more, to the frustration of many residents and businesses.

Transport Minister Andrew Constance has called the subcontractor's claims "ridiculous", saying he is an angry customer and they need to "get on with it".

"We are furious it's not being delivered at the pace it should be," he said.

"We will throw the book at them in terms of this contract. We are not going to muck around.

"The NSW taxpayer is not going to be fleeced by anyone."

But the Opposition said the 24-kilometre line from the CBD to the eastern suburbs could now become the world's most costly light rail project.

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